The present invention relates to a device for switching a coil from a state where a current is forced through it to a state where it delivers a current. The invention more particularly concerns the switching of the coil in a record/play-back head of a video tape recorder.
In play-back mode, one of the terminals of the coil is grounded and the voltage on the other terminal is detected. In record mode, the terminal which was grounded is connected to a current source and the other terminal is connected to a supply voltage, means being provided for modulating the current source and forcing current through the coil as a function of the signal to be recorded.
When it is desired to switch from one mode to another, the coil is subject to two dangers liable to damage it: it can be connected between ground and the supply voltage for a short period, which would cause a high current surge because it has a low impedance, and it can be crossed by a discharge current peak of the coupling capacitor to an amplifier, as will be seen hereafter.
In the prior art, to solve these problems, the devices that have been designed are difficult to integrate and comprise a large amount of switches, the operation of which has to be carefully synchronized by complex processes.
French patent application no. 2 628 877 filed on Mar. 18, 1988 describes a particularly simple switching device, comprising a small number of switches easily integrable. The circuit described in this document has only two switches, but one of these switches, as well as three surrounding components, is not integrated.
An object of the invention is to provide a coil switching device with no external components.